An air interface is used in a wireless communication system to define the exchange of information between user equipment UE, for example a radiotelephone, and a base station or other communication system element.
For example, the High Speed Downlink Packet Access HSDPA specification, which forms part of the third generation partnership project 3GPP universal mobile telecommunication system UTMS specification, defines a High Speed Downlink Shared Channel HS-DSCH for allowing data transmissions from a base station to a plurality of UEs by sharing a given number of HS-DSCH codes among the plurality of UEs in a time/code division manner. To facilitate the sharing of the HS-DSCH channel among a plurality of UEs in a time/code division manner, an associated High Speed Shared Control Channel HS-SCCH provides information that allows a UE to make a determination as to whether data being transmitted in the HS-DSCH is intended for the UE.
As such, HS-SCCHs are used for transmitting signalling information to allow a UE to determine which data transmissions are intended for the UE and to allow the processing of data transmitted on the HS-DSCH by the appropriate UE.
The signalling information (i.e. control channel data) that is incorporated in an HS-SCCH is transmitted in time transmission intervals TTIs, where a TTI is divided into two parts. The first part of a TTI uses a UE specific masking, which allows a UE to make a determination as to whether data transmitted on an associated HS-DSCH is intended for that particular UE. The second part of the TTI includes a UE specific Cyclic Redundancy Check CRC attachment, which makes it possible to assess the result of HS-SCCH detection performed from the first part of the TTI.
The TTIs of the HS-SCCH are built on a three time slot per frame structure corresponding to a time interval of 2 ms. The first part (i.e. Part 1) of the HS-SCCH control channel is transmitted in the first time slot of the TTI and includes information of the HS-DSCH channalization code set (corresponding to 7 bits) and modulation scheme (corresponding to 1 bit). The second part (i.e. Part 2) of the HS-SCCH control channel is transmitted in the second and third time slots of the TTI and contains information on the HS-DSCH transport block size (corresponding to 6 bits) and Hybrid Automatic-Repeat Request HARQ process (corresponding to 7 bits). For robustness and to aid data recovery the data associated with part 1 and part 2 of the HS-SCCH TTI is encoded, using a convolutional code of rate R=1/3 and constraint length K=9.
For the purposes of the 3GPP UTMS standard the coding scheme applied to the signalling information transmitted in Part 1 of the HS-SCCH produces 48 bits from the 8 Part 1 information bits. These encoded bits are then rate matched (i.e. punctured) to produce 40 bits, which are masked by a UE specific mask, thereby generating a 40-bit transmitted codeword as is well known to a person skilled in the art.
This process is illustrated in FIG. 1, which shows a transmitting element 10 for use in a wireless communication system element 11, for example a base station, which is arranged to encode a data string. The transmitting element includes a first element 12 that is arranged to receive the 7 channelization code bits 13 and the modulation code bit 14 to which is appended 8 tail bits. The information bits and appended tail bits are then provided to a convolutional encoder 15, which generates a 48 bit codeword that is fed to a rate matching element 16. The rate matching element 16 punctures the received codeword to produce a 40 bit sequence, which is passed to a masking element 17 that masks the rate punctured codeword with a UE specific scrambling sequence. The resulting codeword is then passed to a transmitter 18 for modulation, spreading and generation of a WCDMA transmitted signal.
To allow a UE to make a determination as to whether there is data being transmitted in one or more of the HS-DSCH codes that is intended for the UE, part 1 of the HS-SCCH (i.e. the first time slot of the HS-SCCH TTI) is transmitted in advance of the HS-DSCH data transmission. As such, a UE must decode the first part of the TTI in each HS-SCCH, where typically in a 3GPP UTMS system there are up to four HS-SCCHs transmitted simultaneously, in order to determine whether or not a data transmission included in a HS-DSCH is intended for that particular UE.
This process is complicated in that the information bits that form the first part of the HS-SCCH do not include a CRC attachment. As such, to aid data recovery a receiver uses convolutional decoder metrics for error detection.
Two common convolutional decoder techniques used for error detection are the Viterbi path metric difference algorithm and the Yamomoto-Itoh YI algorithm.
The Viterbi algorithm is based on a trellis diagram that is used to perform the decoding process in order to identify the particular path through the trellis that maximizes the probability that the corresponding bit sequence was transmitted, conditioned to the received data samples (Maximum Likelihood ML sequence).
In particular, the Viterbi path metric difference algorithm computes the difference in Viterbi path metrics between the merging paths in the last stage of a Viterbi trellis. The calculated difference is compared to a threshold. If the calculated difference is greater that the threshold the decoding is declared a success, otherwise it is declared a failure. When performing this calculation on the first part of the HS-SCCH, a successful decoding implies that the HS-SCCH transmission is estimated to be intended for the UE and a failure implies that the HS-SCCH transmission is estimated not intended for the UE.
An improved technique for decoding the HS-SCCH uses the YI algorithm, where the YI algorithm is based on a modified form of the Viterbi algorithm to produce a reliability indicator. In particular, the YI algorithm is based on the principle that when two paths merge in a Viterbi trellis and are close in terms of their path metrics, then the selection of one of the paths over the other is prone to error. For example, states in a trellis are labelled as “good” or “bad” depending on whether the survivor path at a state is reliable or not. To begin with all states are labelled “good.” As Viterbi decoding progresses and a survivor path is selected over a merging path at a state, the path metric difference is computed. This computed path metric difference is compared to a threshold. If the computed difference is less than the threshold the surviving path is labelled “bad,” otherwise it is labelled “good.” In any subsequent stage in the trellis, if a path labelled “bad” is selected over a merging path it retains the label “bad” even if the path metric difference exceed the threshold at that stage. At the end of the Viterbi decoding, the label on the chosen survivor path is checked. If the survivor path has a “good” label the decoding is regarded as a success and if “bad” the decoding is declared a failure.
However, both the Viterbi and YI algorithms can be computationally intensive, which can result in increased processor requirements and correspondingly an increase in associated power and cost of a device.
It is desirable to improve this situation.